Fractional Brownian motion in crowded fluids

Diffusion in crowded fluids, e.g. in the cytoplasm of living cells, has frequently been reported to show anomalous characteristics (so-called ‘subdiffusion’). Several random walk models have been proposed to explain these observations, yet so far an experimentally supported decision in favor of one of these models has been lacking. Here, we show that experimentally obtained trajectories in a prototypical crowded fluid show an asphericity that is most consistent with the predictions of fractional Brownian motion, i.e. an anti-correlated, anti-persistent generalization of normal Brownian motion that is related to the fluid's viscoelasticity.